. CURRENT PAPERS Designing to Avoid Dangerous Beha

,NAnoNALAERONAUTKAiESTABLISHMENTcpNo66dwll3RAR.y’k7dA.R.C.Technlcall+ortMINISTRYOFSUPPLYAEROtiAUTICALRESEARCHCOUNCIL.CURRENTPAPERStDesigningtoAvoidDangerousBehaviourofanAircraftduetotheEffectsonControlHingeMomentsofIceonthe*.LeadingEdge\oftheFixedSurfaceD.E.Morns,B.Sc.CrownCopyrrghtReservedLONDON:HISMAJESTY’SSTATIONERYOFFICE\1952PriceIs.9d.net,,hAllON&AEROMAUTKALESTABLISHWW!L.BBWARYC.P.lio.66TechnicalNoteNoAem.March,1947ROYALAIRCRAFTESTABLISHME~DesigningtoavoiddangerousbehaviourofanaircraftduetotheeffectsoncontralhingemomentsoficeontheleadzngedgeofthefixedsurfaceDE.Morris,B.Sc.,SUMMARYTheresultsofwindtunnelneasuremsntsofthehingemomentsofaVikingelevatorwithsinmlatediceontheta&planeleadingedgeareusedtoexplainuncontrollablepit&goscillationsofthealrcraftwhichoccurredduringaflightwithiceonthetailplaneleadingedge.Referenceismadetotheoretical.reportswhichshowthatwithcertainelevator(orrudder)hingemomentcharacteristicsincreasingoscilla-tionsinpitch(oryaw)areobtained.Itissuggestedthattheimposingofcertainlimitationsonthecontrolsurfacehingemomentcoefficientsw-illelvninatethepossibilityofarepetitionoftheVikingincidentonfutureaircraftdesigns.InordertoachievethisitISrecommendedthatforelevatorsandruddersthevalueof-b2shouldnotbelessthan0.10(0.12iftheyhaveanunshieldedhornbalance).Foraileronsitisreoorsnendedthat-b2shouldnotbelessthan0.075.Thevalueoftheelevatorblshouldbesuchthatthestickfreeneutralpointisnotmorethan0.05~aftofthestickfixedneutralpoint.Forruddersthevalueofblshouldbedesqnedtobelessthan0.05.ItISsuggestedthattestsshotidbeputinhandtodeternunetheaccuracywithwhichb2foranelevatorcanbemeasuredinflightduringquickroutinetests.Shoulditprovepracticabletoobtaintheneededaccuracy,considerationshouldbegiventoframingadefiniterequirementforalowerlimitfor-b2onelevatorswithpossiblyanescapeclauseincertaincases,e.g.forpoweroperatedcontrols.VerysoonafterthisreportwaswrittenMessrs.Vickers-AnnstrongsLtd.,introducedamodificationtotheVikingelevatorwhich,whilststillmaintarnanganadequatemsrgs.nofstickfreestability,hassuccessfullyCLU-&thetroublesduetoice.LISTOFCONTENTS1Introduction2TunnelmeasuretnsntsofthehingemomentsofaVikingelevator3FlrghtexperienceontheViking4Designv-duesofblandb25PraoticabilityofflighttestsReferencesILISTOFILLUSTRATIONSIceFornationTestedmWind.TunnelTypicalbouidariesofRudderblandb2fordampedoscillationsPs3FlRure(‘Lb)2-2-YI-.I1<rodudlonFollovnngarecentlncdentduringaflightonaVikinga-raftwhen&fflcultywase~perlencedincontrollingpitchingoscillationswhichoccurredwhenxeformedonthetadplaneleadingedge,thequestionhasarisenofwhetherdesignrulescanbefornailatedtopreventfuturealrorafttypessufferingfmmsuchtroubleInthisnoteanattemptwillbemadetoexplasnthereasonsforthebehaviourofthearrcraftintheseconditions,snddesignrulesaresuggestedforclvoi&ngsuchbehaviouru1future.Gu[e-mail]@l.aneanddevatorinwhichelevatorhingemomentsweremeasurealvlthvarioustypes3ficefomationssimulatedonthetadplaneleadingedge.Theresultsofthesetestsshowedthaticeonthetailplaneleadxngedgecaused.comparativelylargechangesintheelevatorlungemomentsu1thedirectionofoverbalance,i.e.blbecam?lessnegative(ormoreposltlve)andb2becamelessnega-tive.Whenconsideringpossiblevardxonsofthebasichingemomentsofcontrdsitisessentialtoconsderthevariationsfromnormalduetorandommanufactuz7.ycrmrs.Theimportanceoftheeffectsofsucherrorshasbeenrealisedforalongtimeanddesignershavebeenwarnedoftheirpossiblemagnituae2.Itisobviouslyimportantthattheeffectsofsuchvariationsincontrolhingemomentsshouldnotbeallowedtocausedangeroushandlingcharacteristicsoftheamcraft.Anobviouslydangerousbingemomentcharacteris&isapositivevalueofb2,i.e.overbalance.ThusItappearsdesirabletodesigntoaminimumvalueof-b2SOttittheeffectsofIcecombinedwiththeeffectsofmanufacturingerrorsdonotmakeb2positive.Ifblislargeandpositiveincombinationmthasmall(nearzero)positiveornegativevaluesofb2fortheelevator(orrudder)undampedstickfreepitching(ordirectional)oscillationsoccur.Theseoscilla-tlonsarecausedbytheinducedmovementsofthecontmlsurfaces.WhentheaircraftisdIsturbed,givingachangeintailincidence,thecorn-parativelylargevaluesofposltlvebl,combinedwithasmallvalueof-b2,causesalargestabilisingcontrolmovemmt.Thisovercorrectsthedisturbanceandthusanincreasingoscillation1ssetup.Thistypeofmotionhasbeeninvestlgate?itheoreticallyforbothpitch3andyad+.Theperiodoftheoscillation1saboutl-2secondsforthepit&goscillationsandaboutl-5secondBforthedxectiond.oscillations.Exam$lesofsuchosclllatlonshavebeenobtainedinflightonquoteanumberofaircraft,evenvnthouticepresent.Ingeneral,however,tliimotionhasnotbeenunclamped.Whenconsideringthepossibilityofthepilotbeingabletostopsuchoscillations,veryimportantfactorsaretheshortnessoftheperiodandthemagnitudeofthestickforcesrequiredtopreventtheinducedcentralmovements.Whenthepselrodisabout1sec.itispossibleforthepilot,inhisattemptstodan?pouttheoscillatimtogeto&ofstepandactuallymovehiscontrolsoastomu-easetheoscillation.Themostsatisfactorywayofdealingvvlthsuch-oscillat~o~Sap